1. Field of the Invention
The present invention relates to a thin film transistor array panel using polysilicon as a semiconductor.
2. Description of Related Art
A thin film transistor (TFT) array panel is used as a circuit substrate to individually drive each pixel in a flat panel display having a plurality of pixels such as a liquid crystal display or an organic light emitting diode display.
The liquid crystal display (LCD) includes two panels provided with field-generating electrodes such as pixel electrodes and a common electrode, and a liquid crystal (LC) layer interposed therebetween. The LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the LC layer, which determines orientations of LC molecules in the LC layer to adjust polarization of incident light.
The organic light emitting diode display (OLED) is a self emissive display device, which displays images by exciting an emissive organic material to emit light. The OLED includes an anode (hole injection electrode), a cathode (electron injection electrode), and an organic light emission layer interposed therebetween. When the holes and the electrons are injected into the light emission layer, they are recombined and pair annihilated with emitting light.
Each pixel of the OLED includes two transistors such as a drive transistor, typically a TFT, and a switching transistor, typically a TFT. The current for light emission is driven by the drive TFT and the amount of current driven by the drive TFT is controlled by the data signals from the switching TFT.
Typically, the TFT array panel includes a semiconductor made of amorphous silicon or crystalline silicon. Amorphous silicon is widely used in displays having glass with a low melting point, since amorphous silicon film can be fabricated at low temperatures.
However, the amorphous silicon film has low carrier mobility. It may be unsuitable for applying to a high quality drive circuit of display panels. Whereas, since polycrystalline silicon has good electric field effect mobility, then in high frequency operation, and with low leakage current, high quality drive circuits, polycrystalline silicon is more desirable.
The electrical characteristics of the TFT array panel using polycrystalline silicon are influenced by the size and the uniformity of the grain. That is, the electric field effect mobility of the TFT array panel is increased according to the increased size and uniformity of the grain. The method of forming the polycrystalline silicon with increasing the size and the uniformity of grain is of concern.
Excimer laser annealing (ELA) and chamber annealing are typical methods for producing polycrystalline silicon. Recently, a sequential lateral solidification (SLS) process deriving lateral growth of the silicon crystalline has been proposed.
The SLS technique utilizes a phenomenon where the silicon grains grow laterally to the boundary of a liquid region and a solid region. In the SLS process, the size of the grains can be as much as the predetermined widths by controlling the irradiation range and the energy of a laser beam using an optic system and a mask selectively passing the laser beam.
The ELA technique utilizes an excimer laser as a source and a sequential shifting of irradiation region to crystallize the amorphous silicon into the polycrystalline silicon. In the ELA, the laser irradiation is sequentially executed, and the portion of the irradiation region is overlapped by aligning the irradiation region of the excimer laser. Accordingly, the excimer laser is repeatedly irradiated on the predetermined region.
After sequential lateral solidification, protrusions are formed on the surface of the polysilicon layer along the grain boundaries due to the grains growing toward each other. These methods reduce the flow of current, and result in degrading the characteristics of the TFTs, thereby causing defects such as horizontal stripes or vertical stripes.
Furthermore, the difference in the shifting pitch and the beam width are generated after excimer laser annealing, and the number of irradiation cycles is changed depend on the positions of the semiconductor. As a result, the crystallized state of the semiconductor causes the stripe defect.